Sorry if my question is silly, but I'm not really good with electronics.

I have to power a device with 3V and 200 mA. So the 3.3V output pin of the arduino seems great, and in fact I can power my external device which runs fine.

However, I have to switch the power to that device via software, so I thought a transistor would help me. I connected the 3.3V to the emitter, one of the arduino pins to the base, and the collector goes to my device.

The fact is with this configuration I got too much voltage or current to my device, which doesn't work well. How can I switch the 3.3V from on to off? Do I have to use the transistor? If yes, how can I use it to make it pass only 3.3V and 200 mA?

Sorry if my question is silly, but I'm not really good with electronics.

I have to power a device with 3V and 200 mA. So the 3.3V output pin of the arduino seems great, and in fact I can power my external device which runs fine.

The arduino product page for Uno still only rates the 3.3v pin as having 50ma capacity. This probably a legacy spec when the FTDI was supplying the 3.3v pin on older arduino boards. The Uno uses a dedicated 3.3vdc voltage regulator to supply that pin, but I would think it's safe ratings is around 100ma or so, so you may be really stretching it drawing 200ma. But if it works it works I guess.

However, I have to switch the power to that device via software, so I thought a transistor would help me. I connected the 3.3V to the emitter, one of the arduino pins to the base, and the collector goes to my device.

NPN or PNP transistor? No resistor on the base is a good way to damage both the transistor and the output pin.

The fact is with this configuration I got too much voltage or current to my device, which doesn't work well. How can I switch the 3.3V from on to off? Do I have to use the transistor? If yes, how can I use it to make it pass only 3.3V and 200 mA?

Sorry if my question is silly, but I'm not really good with electronics.

I have to power a device with 3V and 200 mA. So the 3.3V output pin of the arduino seems great, and in fact I can power my external device which runs fine.

What device? A transistor will eat up 0.7V so you'll really only be getting 2.6V across the device if you use the 3.3V pin. It might be better to use 5V and add a resistor to eat up the excess 1.3V @ 200mA (use Ohm's Law to find the resistance needed for that...)

No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

However, I have to switch the power to that device via software, so I thought a transistor would help me. I connected the 3.3V to the emitter, one of the arduino pins to the base, and the collector goes to my device.

That is the way to configure a PNP transistor only. These are not the most common type. You always need a resistor between the base and the arduino pin.Please say what number the transistor is so we can check if you have it wired correctly.

If you have an NPN type transistor ( BC337 ), and need to switch a 200mA 3.3V device, you connect emitter to GND, collector to the - side of your device, + side of the device to 3.3Vand a resistor of <=1k between Arduino pin and base.

Hope there's no need to have your device - directly connected to GND. (What is it ?)